U.S. patent application number 14/473557 was filed with the patent office on 2016-03-03 for rotary solar converter.
The applicant listed for this patent is Paul Wilkinson Dent. Invention is credited to Paul Wilkinson Dent.
Application Number | 20160065090 14/473557 |
Document ID | / |
Family ID | 54147263 |
Filed Date | 2016-03-03 |
United States Patent
Application |
20160065090 |
Kind Code |
A1 |
Dent; Paul Wilkinson |
March 3, 2016 |
ROTARY SOLAR CONVERTER
Abstract
An advantageous method of converting solar energy from a
photovoltaic array into alternating current for feeding into the
electricity grid is described based on the use of an inventive
rotary machine. The inventive rotary machine has a rotor and a set
of stator coils which are excited in a first mode by a polyphase
current derived from the solar array and simultaneously in a
second, orthogonal mode by a polyphase voltage derived from the
electricity grid.
Inventors: |
Dent; Paul Wilkinson;
(Pittsboro, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dent; Paul Wilkinson |
Pittsboro |
NC |
US |
|
|
Family ID: |
54147263 |
Appl. No.: |
14/473557 |
Filed: |
August 29, 2014 |
Current U.S.
Class: |
363/102 |
Current CPC
Class: |
H02P 25/22 20130101;
H02K 47/26 20130101; H02K 17/02 20130101; H02K 17/42 20130101; H02M
7/54 20130101; Y02E 10/56 20130101 |
International
Class: |
H02M 7/54 20060101
H02M007/54 |
Claims
1. A rotary electrical machine for transferring electrical power
from a Direct Current (DC) power source to an Alternating Current
(AC) electricity grid, comprising: An induction motor driven from
said DC source by a polyphase switching circuit to produce rotation
of the rotor; A synchronous induction generator coupled to said
electricity grid to transfer power to said AC electricity grid,
wherein said induction motor and said induction generator use the
same stator, the same stator coils and the same rotor.
2. The rotary electrical machine of claim 1 wherein said polyphase
switching circuit generates a first number of phases N and the
connection to said electricity grid provides a second number of
phases M, and the number of said stator coils is a multiple of both
N and M
3. The rotary electrical machine of claim 1 wherein said polyphase
switching circuit produces a number of squarewave drive voltage
waveforms which are equispaced in phase and having a frequency that
is approximately a multiple of or a sub-multiple of the frequency
of said electricity grid, said stator coils being connected to the
polyphase drive voltages in a first way and to said electricity
grid in a second way such that no current at the squarewave drive
frequency is injected into the electricity grid.
4. The rotary machine of claim 1 in which said DC power source is a
photovoltaic solar array.
5. The rotary machine of claim 1 in which said polyphase switching
circuit is controlled to produce polyphase drive signals of a
controlled frequency slightly higher than a multiple of or a
sub-multiple of the frequency of said electricity grid, the
frequency being controlled to optimize the power transferred from
said DC source to said electricity grid.
6. The electrical rotary machine of claim 1 in which said DC source
is electrically floating.
7. A rotary electrical machine comprising: A stator lamination
stack of magnetic material; a rotor disposed within said stator and
free to rotate on bearings; a number N of stator coil windings
disposed at regular angular spacings within slots in said stator
lamination stack, wherein the stator coils are excited
simultaneously by a first polyphase electrical excitation having a
phase increment of M1.times.360/N degrees and a second polyphase
electrical excitation having a phase increment of M2.times.360/N
degrees and integers M1 and M2 are chosen such that electrical
coupling between the first and the second polyphase excitations is
substantially zero.
8. The rotary electrical machine of claim 7 in which said first
polyphase excitation uses two-level waveforms and said second
polyphase excitation uses sinusoidal waveforms.
9. The rotary electrical machine of claim 7 in which said first
polyphase excitation is produced by a switching circuit that
connects one end of each of said stator coils alternately to the
negative or the positive line of a DC source in a predetermined
pattern.
10. The rotary electrical machine of claim 7 in which N is an odd
integer and said first polyphase excitation comprises 3-level
electrical waveforms.
11. A method of converting Direct Current to Alternating current
for feeding into the electricity grid, comprising: Generating a set
of non-sinusoidal motor drive waveforms to drive the stator coils
of an induction motor; connecting said same stator coils to one or
more service drop transformers of said electricity grid such that
Alternating Current having a sinusoidal waveform is transferred
into said electricity grid and substantially none of said
non-sinusoidal motor drive waveforms is coupled into said
electricity grid.
12. The method of claim 11 in which generating a set of
non-sinusoidal motor drive waveforms comprises generating a
polyphase set of squarewave waveforms which are relatively
time-shifted by sub-multiples of their repetition period.
13. The method of claim 11 wherein connecting said same stator
coils to one or more service drop transformers comprises connecting
one end of each of a subset of said stator coils to a hot leg of a
utility transformer and the other end of each of said subset of
stator coils is driven by a respective one of said non-sinusoidal
motor drive waveforms, wherein the set of non-sinusoidal motor
drive waveforms that drive said subset of stator coils has the
property that their sum is nominally zero at all times during each
cycle.
14. The method of claim 11 in which the repetition frequency of
said non-sinusoidal drive waveforms is slightly higher than an
integer multiple of the frequency of said electricity grid by a
controlled slip rate.
15. The method of claim 11 in which the frequency of said
non-sinusoidal drive waveforms is slightly higher by a controlled
slip rate than the frequency of said electricity grid divided by an
integer.
16. A method of feeding current and transferring power into the
electricity grid from an energy source, comprising: generating
electrical drive signals using said energy source at a frequency
substantially different than the frequency of said electricity
grid; connecting said electrical signals and said electricity grid
to the same stator coils of a rotary electrical machine having a
single stator and a single rotor such that current is fed into the
electricity grid at its own frequency thereby achieving said
transfer of power.
17. The method of claim 16 in which said energy source comprises a
photovoltaic array.
18. The method of claim 16, further comprising controlling the
frequency of said electrical drive signals to optimize said
transfer of power.
Description
BACKGROUND
[0001] The invention relates to DC to AC conversion methods, and in
particular to methods to convert DC power from a solar array into
AC power for feeding directly into the electricity grid.
[0002] Various methods for DC to AC conversion are known in the art
and new methods continue to be invented, as described in allowed
U.S. patent application Ser. No. 13/103,070 to current inventor. In
the art prior to solid state switching converters, it was known to
use rotary converters to convert electrical power of one type into
electrical power of another type. A rotary converter of the prior
art comprised a motor driven by electrical power of one type at its
input, the motor being mechanically connected to drive a generator
to produce electrical power of another type at its output. Prior
art rotary converters were known in which the motor and generator
used separate rotors and field coils, and types were also known in
which the motor and generator used the same stator coils and the
same rotor, the rotor being wound with a motor winding connected by
brushes to a DC input source and a generator winding connected
through brushes and a commutator (for DC output) or slip rings (for
AC output) to a load. When the input power was at one DC voltage
and the output power was at another DC voltage, the rotary
converter was also known as a dynamotor. When the input power type
was DC and the output power type was AC, the rotary converter was
also known as an Inverter. Rotary Inverters were commonly used in
aircraft to convert 28 volts DC to 115 volts AC at 400 Hz, but have
largely been replaced by solid state inverters in modern
aircraft.
[0003] The prior art also includes a type of rotary converter for
producing 3-phase power from single-phase power: A single-phase
induction motor has additional windings from which a second and
third phase can be derived. This type of converter is characterized
by AC in and AC out that both comprise sinusoidal waveforms at the
same frequency.
[0004] Neither a motor nor a generator is 100% efficient, therefore
the efficiency of a motor-generator combination is the product of
the efficiencies of the motor and the generator respectively. For
example, if the motor converts DC power to mechanical rotational
energy with an efficiency of 75%, and the generator converts
rotational energy to AC electrical output power with an efficiency
of 80%, then the combined efficiency of DC to AC conversion is
80.times.75=60%. Low conversion efficiency was thus a
characteristic of prior art rotary converters having separate motor
and generator sections. Dynamotors and rotary inverters with a
common rotor and stator also tended to have low efficiency due to
brush friction, brush voltage drop and field power requirements, as
well as the fact that having input and output windings on the same
rotor limits the gauge of wire that can be used for each.
[0005] Rotary inverters have several advantages however; rotary
inverters can produce clean, pure sinewave output voltage
waveforms; rotary inverters can handle and withstand short periods
of high overload due to the inertia of the rotor; rotary inverters
can easily produce one, two, three or more output phases and rotary
inverters have the potential to be of lower cost than solid state
inverters in certain higher power ranges. Other advantages of a
rotary inverter in solar energy applications will become apparent
upon reading the description herein of the invention. A rotary
inverter with improved efficiency, comparable to a solid state DC
to AC inverter, can therefore provide an advantageous alternative
to purely solid state inverters.
SUMMARY
[0006] A mechanical, rotary DC to AC inverter is described in which
the motor and generator sections are merged into a single
induction-type rotor and single set of stator coils, there being
thereby no mechanical energy output required from a motor shaft and
no mechanical energy input required to a generator shaft.
[0007] The number of stator poles or coils N is not prime and can
be connected to a polyphase AC source in as many different
orthogonal modes as there are factors of N.
[0008] In an exemplary implementation, N=6, and the six stator
coils are connected to a 3-phase electric utility grid of frequency
F in the phase progression 0, 120, 240, 0, 120, 240 degrees while
simultaneously being connected to a source of power at frequency
F/2+dF in the phase progression 0, 60, 120, 180, 240, 300 degrees,
the rotor turning at a frequency of approximately (F+dF)/2
revs/sec.
[0009] When the slip frequency dF is positive, this results in
power being transferred from the source of frequency F/2+dF to the
utility grid at a frequency F and with a pure sine waveform
substantially irrespective of the waveform of the source of
frequency F/2+dF, which may therefore be a two-level switching
waveform. Other values of N and other orthogonal modes are also
described.
[0010] In an inventive solar farm application, the source of
frequency F/2+dF is derived from a solar array by on/off switching
transistors which do not produce a pure sinewave, and a control
circuit adjusts dF such that the maximum amount of power is
transferred from the solar array into the utility grid with a pure
sine waveform.
[0011] In a preferred implementation, N=12, and the machine is
excited by a 12-phase square-wave source with power derived from a
solar array and is excited by 6-phases (3-phases and their
inverses) from the electricity grid, the 12-phase solar-derived
source operating at half the grid frequency plus a slip frequency
which is controlled to deliver the maximum power from the solar
array to the electricity grid.
[0012] Due to the generator operation of the invention being of the
synchronous, induction type, the invention automatically fulfills
the requirement to stop feeding power to the grid should the grid
fail.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows a prior art motor-generator combination
[0014] FIG. 2A shows a 6-pole rotary machine stator excited in a
first mode
[0015] FIG. 2B shows a 6-pole rotary machine stator excited in a
second mode
[0016] FIG. 3 shows a 6-pole rotary machine stator excited
simultaneously in two orthogonal modes
[0017] FIG. 4 shows a transistor switching arrangement for
generating 3-phase, non-sinusoidal power waveforms.
[0018] FIG. 5 shows magnetic field lines in a rotor when excited
with two orthogonal excitations.
[0019] FIG. 6 shows an exemplary circuit for exciting a rotary
machine in two orthogonal modes simultaneously.
[0020] FIG. 7 shows one way of winding the stator coils in the
stator lamination slots for a 6-pole machine.
[0021] FIG. 8 shows one way of winding the stator coils in the
stator lamination slots for a 12-pole machine.
[0022] FIG. 9 shows a general method of exciting an N.times.M pole
machine in an N-phase mode and an orthogonal M-phase mode.
[0023] FIG. 10 shows another general method of exciting an
N.times.M pole 105 machine in an N-phase mode and an orthogonal
M-phase mode when M is even.
[0024] FIG. 11 shows the preferred stator coil connections for a
12-pole machine.
[0025] FIG. 12 shows the waveforms of a 3-level, 5-phase excitation
wherein the five phase waveforms sum to zero at every time
instant.
[0026] FIG. 13 shows the connections of a 12-pole machine to a
single-phase electricity grid.
DETAILED DESCRIPTION
[0027] FIG. 1 illustrates a prior art motor-generator combination
in which the motor and generator share the same rotor and stator.
The stator comprises magnetic pole pieces 30 disposed in opposite
pairs around rotor 10. In the prior art, the stator field was
produced by an electromagnet using stator coils (not shown), the
field coil power being a first factor contributing to loss of
efficiency. If such a device were to be manufactured using modern
technology, neodymium permanent magnets would preferably be used to
produce the field.
[0028] Rotor (1) is formed by stacking star-shaped, iron
laminations on a shaft. The stack of rotor laminations forms slots
to hold the input rotor windings (40) and the output rotor windings
(20). Because the rotor slot volume has to accommodate both input
and output windings, the wire cross sectional area available for
each winding is only half of that which otherwise could have been
used for a single winding. The resistance of each of the input and
output windings is thus around double (or more due to the need for
insulation) that of a single winding occupying the same slots,
which is a second factor contributing to loss of efficiency
[0029] When the input is DC, the prior art supplied the DC to the
input windings through a commutator and carbon brush arrangement.
When the required output was also DC, the output windings supplied
the load through a commutator and brush arrangement, else for an AC
output, through slip-rings and brushes. The frictional and
electrical losses of the brushes and commutators or slip rings is a
third factor contributing to loss of efficiency.
[0030] FIGS. 2A and 2B illustrate an arrangement of six stator
coils for a 6-pole induction machine. In an induction machine,
(motor or generator) power input goes to or power output comes from
the stator coils. The rotor comprises a revolving magnetic core as
in FIG. 1, except that the rotor winding is simply a short circuit
formed by one or more closed loops of copper bar, and no connection
to the rotor is needed by means of brushes and commutators or slip
rings. Some of the aforementioned sources of efficiency loss in the
machine of FIG. 1 are thus avoided.
[0031] The six stator coils (200) of FIG. 2A may be excited by
alternating current in the phase progression 0, 120, 240, 0, 120,
240 degrees. Because the phase goes through two cycles around 360
degrees of mechanical rotation, a rotating magnetic field is
produced at half the AC frequency. For example, if the AC frequency
is 60 Hz, the magnetic field rotates at 30 revolutions per second
and drags the rotor around at that speed, that is 1800 RPM. If the
rotor revolves at exactly that speed, moving around with the
magnetic field, it experiences a static magnetic field in its
rotating frame of reference and thus does not experience a torque
force. A torque is required if the shaft is to drive a load, and in
that case, the rotor revolves at slightly less than 1800 RPM, the
difference being called the slip rate. This results in the rotor
containing a static magnetic field in the 1800 RPM-rotating frame
of reference, misaligned by 90 degrees to the stator field in the
same frame of reference. The attempt by these misaligned magnetic
fields to align themselves produces the torque to turn the rotor
against the load.
[0032] If on the other hand a torque is supplied to the shaft that
attempts to turn the rotor at a greater rate than 1800 RPM, the
rotor will experience a magnetic field rotating in the opposite
direction in its rotating reference frame, thus causing drag
opposing the torque. The energy supplied to turn the shaft against
this drag appears as power flowing back to the exciting source, and
this is the principle of the synchronous induction generator. An
induction generator only generates power in this way when connected
to a pre-existing source of excitation, such as the electricity
grid. If the source of excitation vanishes, the generator stops
generating power as there is no longer any magnetic field created
by the stator coils. This renders the induction generator a
favorite for wind turbines that feed power to the grid, as they
will stop feeding power to the grid if the grid fails, as required
by regulations. Somewhere in the grid system, there must be a
source that is not an induction generator, such as a generator with
an independent field excitation, as used in electric utility power
stations.
[0033] FIG. 2B shows that the same six stator coils can be excited
in the phase progression 0, 60, 120, 180, 240, 200 degrees, which
goes through only one cycle in one mechanical revolution. This
produces a magnetic field that rotates at the same frequency as the
excitation source, namely 60 revolutions per second for 60 Hz or
3600 RPM. On the other hand, using 30 Hz excitation in this mode
would produce 1800 RPM, as when using 60 Hz in the mode of FIG.
2A.
[0034] FIGS. 2A and 2B are merely illustrative of 6 stator coils,
and are not intended to imply that a coil spans only two poles.
FIGS. 7 and 8 illustrate winding configurations in which each
stator coil spans more than one pole, and in which adjacent coils
have some overlap. All of the known art of efficient polyphase
motor or generator design may be applied in choosing a most
appropriate winding configuration for the invention, and many text
books and academic papers on the subject have been written and are
readily available. The modern method of finite element analysis may
also be applied to optimize the design of such rotating electrical
machines when configured according to the invention.
[0035] It will be appreciated that a phase of 60 degrees is 180
degrees removed from a phase of 240 degrees, such as shown as phase
L3(a) in FIG. 2A. Therefore the current for the 60-degree coil of
FIG. 2B is produced by reversing the coil connections to a phase of
240 degrees. Likewise, a phase of 300 degrees is simply a phase of
120 degrees with the coil connections reversed.
[0036] FIG. 3 illustrates how the six stator coils can be excited
in both modes simultaneously, using 60 Hz for the mode of FIG. 2A
and 30 Hz for the mode of FIG. 2B to produce the same 1800 RPM for
each. The two 0 degree stator coils excited with L1(a) are joined
in series so that the same current flows through both from the
L1-phase utility transformer 1010. There is a utility transformer
for each of phases L1, L2 and L3 which drop the high voltage used
for electricity distribution, typically 13200 volts, to, for
example, a split-phase 120-0-120 supply on the low voltage side,
having 240 volts between the ends and a grounded neutral in the
middle. The L2 and L3 utility transformers feed the L2(a) 120
degree phase coils and the L3(a) 240 degree phase coils likewise.
The terminals on either side of the neutral on the secondary (LV)
side of the utility transformers are generally called "hot legs".
The two hot legs of a single, split-phase service, as is normally
supplied to residential customers, are normally denoted by L1, L2
while a 3-phase 120/208 volt service has three hot legs denoted L1,
L2, L3. In FIG. 3, three split-phase drops are used, each coming
from a separate one of the three grid phases, so that six hot legs
and six phases are available.
[0037] The junction where the two L1(a) 0-degree coils are series
connected and the center tap of the L1 utility transformer, which
is also neutral or ground, provide a pair of terminals into which
the 30 Hz L1(b) excitation may be fed. The current in the two L1(a)
0 degree coils will now be flowing in the opposite direction in one
coil compared to the other for the L1(b) excitation. Likewise the
L2(b) excitation is applied to the junction of the two L3(a) coils
and the L3(b) excitation is applied to the junction of the two
L2(a) 120 degree coils, these connections as shown in FIG. 3
ensuring that the directions of rotation corresponding to the 30 Hz
(b) excitation and the 60 Hz (a) excitation are the same.
[0038] The L1(b), L2(b) and L3(b) excitation phases are derived by
switching transistors commutating a floating DC input source. The
switching transistors do not need to produce a sine wave, and may
produce a square wave with three phases. When either the DC source
is floating or the neutral of the utility transformers is not
grounded, or both, the (b) excitation may be regarded as a 3-wire
"open WYE" connection. If on the other hand the DC source is
balanced about ground and the neutral of the utility transformers
is grounded, then the (b) excitation is a 4-wire, 3-phase WYE
connection having a neutral and three hot legs.
[0039] With the 30 Hz excitation exactly half the frequency of the
60 Hz excitation, and there being no mechanical load on the rotor
shaft, the slip rate would be negligible and the rotor would rotate
at substantially exactly 1800 RPM. In principle, no current or
power would flow to or from either excitation source, except for
reactive current and power required to fund iron losses.
[0040] If now the 30 Hz source is increased in frequency by dF, a
rotor moving at 1800 RPM would now experience a magnetic field
rotating at frequency dF in the rotor's rotating frame of
reference, which thereby attempts to drag the rotor around at the
higher speed of 1800+60 dF RPM. If the rotor were to turn at that
rate however, it would now experience a magnetic field due to the
60 Hz excitation rotating at 60 dF RPM in the opposite direction,
causing drag, and with power flowing to the utility grid instead of
from the grid. A balance between torque due to the 30 Hz+dF
excitation and the drag caused by the slower speed 60 Hz excitation
is reached when the rotor turns at somewhere in the region of
1800+60 dF/2, having thus a negative slip rate around -dF/2
relative to the 30+dF Hz excitation and drawing power therefrom,
while having a positive slip rate around +dF/2 relative to the 60
Hz grid excitation, and delivering power thereto.
[0041] The two modes of FIGS. 2A and 2B are orthogonal, due to
diametrically opposite stator coils being fed in phase in one mode
and 180 degrees out of phase in the second mode. Thus when feeding
the machine with both modes simultaneously, none of the L1(b),
L2(b), L3(b) excitation is fed into the utility grid; in effect,
each diametrically opposite pair of stator coils forms a balanced
bridge with the center-tapped utility transformer. The L1(b),
L2(b), L3(b) excitation may therefore depart from a sinewave
without causing the current fed to the utility to depart from a
sinewave. For example, the L1(b), L2(b), L3(b) excitation may be a
squarewave produced by a simple arrangement of switching
transistors
[0042] FIG. 4 shows more internal detail of unit (1000) of FIG. 3,
and the switching transistor arrangement for producing a 3-phase
squarewave excitation. Since the switching frequency is very low,
for example the above-mentioned 30 Hz, the transistors can be very
large area MOSFETs giving negligible voltage drop and negligible
switching losses. Mechanical switches could even be used at such a
low switching frequency.
[0043] Control unit (2000) produces drive signals for the six
MOSFETs Q1 to Q6. At the start of a cycle, Q1 is controlled to
conduct while Q4 is off, making signal L1(b) positive and equal to
the positive DC supply voltage. Likewise Q2 is off and Q5 is on,
making signal L2(b) equal to the negative DC supply voltage. Q3 is
on and Q6 is off so that L3(b) is also positive. After 1/6th of a
cycle, Q3 is turned off and Q6 turned on sending L3(b) negative.
After another 1/6th of a cycle, Q2 turns on with Q5 off, sending
L2(b) positive. Control unit (2000) continues to switch the
transistors on and off in a sequence to produce the three indicated
squarewaves, which are 1/3.sup.rd of a cycle or 120 degrees of
phase apart.
[0044] Input filter (2010) is a low pass filter to prevent at least
high frequency switching transients being exported to the DC input
(2020). Filter (2010) may comprise capacitors connected between the
DC+ve and the DC-ve as well as capacitors connected to the
neutral/ground (1020) of FIG. 3. If the latter capacitors are large
such that the DC+ve and DC-ve voltages are prevented from changing
at the low switching frequency, then each of the L1(b), L2(b),
L3(b) waveforms is effectively referenced to a mean voltage around
zero and thus constitutes a 4-wire, 3-phase WYE connection to the
rotary machine stator coils. On the other hand, if the capacitors
to neutral/ground are small, allowing the DC-ve and DC+ve voltages
to change during each cycle according to a common-mode ripple
waveform, then the 3-phase output is effectively a 3-wire open-WYE
connection. The choice of one or the other is not critical and may
be determined empirically with an actual machine for best
performance. In the case of the open WYE mode, a common mode signal
on the DC input can be useful in detecting ground faults, as
described in allowed U.S. patent application Ser. No. 13/103,070 to
current Applicant, which is hereby 280 incorporated by reference
herein in its entirety.
[0045] If the DC source is strictly balanced relative to ground,
rather than floating as assumed in the preceding paragraph, then
the machine is also being fed with a 4-wire, 3-phase WYE source.
Since it is more difficult to produce a strictly balanced, bipolar
DC source from solar arrays, allowing the DC source to float is
appropriate when the source is a solar array, and thus the open WYE
mode is preferred.
[0046] A brief outline of the theory of operation of the inventive
rotary machine will now be given.
[0047] When the magnetic circuits are operated in the linear region
of the magnetic core material's B-H curve, that is below
saturation, the principle of superposition applies; that is, the
magnetic flux density vector field B due to applying two
magnetizing force excitation vector fields H simultaneously is
equal to the vector sum of the B-fields that would have been
obtained by applying each of the H fields alone, one at a time.
[0048] FIG. 5 illustrates the two sets of field lines that would be
created by applying each of the excitations of FIG. 2 one at a
time. The shapes of the field lines in FIG. 5 are purely
illustrative as in reality they depend on the number, size and
shape of the rotor winding slots.
[0049] The six stator poles (201), when excited by excitation (a)
of FIG. 2, take on magnetic polarities NSSNSS respectively around
360 degrees and at a particular time in the AC cycle of excitation
(a), where the bold letter N indicates a magnetic strength that is
double that of the S poles. The magnetic field lines produced by
this excitation in rotor (10) are indicated by dashed lines.
[0050] When the stator poles are excited by excitation (b) of FIG.
2, the pole polarities are NNSSSN at a particular time of the AC
cycle of excitation (b), and the field lines are shown solid.
[0051] The magnitude of a field indicated by the field lines of
FIG. 5 remains very nearly constant while rotating if the
associated 3-phase excitation is sinusoidal, which fact derives
from the trigonometric identity COS.sup.2+SIN.sup.2=1. On the other
hand, the magnitude of the field lines does not remain constant
under rotation when the excitation is not sinusoidal, such as a
square wave.
[0052] The total field when both excitations are applied
simultaneously is the vector sum of the solid and the dashed
fields, at least in the linear domain of the magnetic material's
B-H loop. In fact, if the excitations are voltage sources, the flux
density produced by each is proportional to the time integral of
the coil voltages, irrespective of non-linearity of the B-H loop.
Notwithstanding this fact however, it is undesirable to drive the
magnetic material into the saturation region of the B-H loop as
this causes excessive current peaks as well as hysteresis loss.
[0053] If excitation (b) was at exactly half the frequency of
excitation (a), the two fields would rotate at the same rate and
the sum field would be of constant shape. Due to the need for a
slip rate to achieve power transfer however, the frequency of
excitation (b) is slightly higher than half that of excitation (a),
so the solid field lines rotate relative to the dashed field lines
producing a sum field that is of a shape that varies cyclically at
the difference frequency F(b)-F(a)/2.
[0054] A voltage is induced in a stator coil by a time-changing
magnetic field. The magnetic field can change either due to the
magnitude of the rotor field changing or due to its direction
changing by virtue of its rotation. The total voltage induced is
due to the sum of the changes in amplitude and/or direction of both
the solid and dashed magnetic field lines. However, whether by
amplitude change or direction change, the voltage induced by
changes in the magnetic field due to excitation (b) are equal and
opposite in diametrically opposite stator coils. Since
diametrically opposite stator coils are connected in series to an
associated utility transformer (see FIG. (3)), the voltages fed
back to the utility due to changes in amplitude or direction of the
magnetic field due to excitation (b) cancel. Thus it is immaterial
if the amplitude of the solid field varies, and thus the associated
excitation (b) need not be sinusoidal. The voltages and currents
fed back to the utility transformers thus remain sinusoidal (at
e.g. 60 Hz) despite the driving voltages from the switching
transistor arrangement of FIG. 4 being square waves (at e.g. 30+dF
Hz).
[0055] If, in FIG. 3, the connections to one of each pair of
diametrically opposite coils is reversed, then the utility
transformers excite the stator coils in the phase progression 0,
60, 120, 180, 240, 300 while the second excitation excites the
stator coils in the phase progression 0, 120, 240, 0, 120, 240, as
shown in FIG. 6. To achieve this while maintaining the same
direction of rotation, L2(a) and L3(a) have to be interchanged as
well as L2(b) and L3(b).
[0056] There are also other ways to connect the stator coils to
swap the phase progressions of the (a) and (b) excitation. For
example, if the connections to both L3(a) coils are reversed, then
it is simply necessary to reverse the connections to the L3 utility
transformer to maintain the same phase progression. Of course it is
not necessarily important to maintain a particular direction of
rotation, so there are other ways to connect the stator coils
together and to the utility transformers which will give the
desired phase progressions or the reverse progressions.
[0057] When the (a) and (b) phase progressions are swapped, the 60
Hz utility current now excites a mode which undergoes a single
cycle of phase progression around 360 degrees, so the machine of
FIG. 6 will rotate at 60 revolutions per second or 3600 RPM. Now
the L1(b), L2(b), L3(b) excitation must be at at frequency of twice
the utility or 120 Hz to correspond to the same speed of rotation,
and the control unit of FIG. 4 must be sped up to four times the
switching frequency of FIG. 4 to generate a 3-phase square wave
excitation at 120 Hz.
[0058] Whether the connections of FIG. 3 or FIG. 6 are used depends
on whether a particular machine design works best with the L1(b),
L2(b), L3(b) excitation at 30 Hz or 120 Hz. Factors which can
influence the choice are an increased reactive current to be
supplied by the switching transistors when 30 Hz is used versus the
increased magnetic losses if the higher frequency of 120 Hz is
used. For the same number of turns on the stator coils, the voltage
required for the 120 Hz excitation mode of FIG. 6 will also be
higher than for the 30 Hz excitation mode of FIG. 3.
[0059] When the arrangement of FIG. 6 is used, current due to
excitation (b) flows in the low voltage side of the utility
transformers. Even though it is flowing in opposite directions in
the two halves of each transformer and thus cancels, it
nevertheless causes additional heating in the windings, which must
therefore be over-dimensioned. To avoid this, an alternative
circuit is shown in FIG. 11. Before discussing FIG. 11 however, two
general arrangements will be described for exciting the stator
coils of a N.times.M pole rotary machine simultaneously with an
N-phase and an orthogonal M-phase excitation.
[0060] FIG. 9 show 15 stator coils 200 fed at one end with 3
repetitions of a 5-phase excitation denoted by L1(b), L2(b), L3(b),
L4(b), L5(b). As is evident from FIGS. 7 and 8, the number of
stator coils and independent magnetic poles does not necessarily
correspond to the number of winding slots in the laminations. As is
known in the art, a single winding may be distributed between
multiple slots in the stator laminations in order to shape the
angular distribution of magnetic field and thus produce fewer
harmonics in the case of a generator.
[0061] The ends of the coils to which the 5-phase excitation is
applied are called the outer ends to distinguish them from the
other ends, which are called. the inner ends. The inner ends of one
each of an L1(b), L2(b), L3(b), L4(b), L5(b) coil are then
connected to one phase of a 3-phase excitation L1(a), L2(a), L3(a).
Because there are three groups of coils each having an L1(b),
L2(b), L3(b), L4(b), L5(b) excitation, the other ends of each group
can be connected to a different one of the 3-phase excitations
L1(a), L2(a), L3(a).
[0062] There are essentially two ways in which one each of an
L1(b), L2(b), L3(b), L4(b), L5(b) coil can be selected to form
three groups. In one arrangement, the inner ends of each group of
five adjacent coils are connected to form the three groups. These
groups would be fed at their inner ends with L1(a), L2(a) and L3(a)
respectively going clockwise. This produces a machine which would
rotate once for every cycle of the (a) excitation, i.e. 3600 RPM
for a 3-phase (a) excitation of 60-Hz. The alternative is shown in
FIG. 9, wherein the inner end of an L1(b) coil is connected to the
inner end of an L2(b) coil spaced 6 away clockwise and then
successively to an L3(b), L4(b) and L5(b) coil spaced likewise,
thereby forming the solid 5-pointed star connection pattern. This
connection receives the L1(a) excitation. The dotted and the dashed
5-pointed star connection patterns connect the inner ends of the
remaining coils to the L2(a) and L3(a) excitations respectively.
Now it may be seen that the 3-phase excitation of the 15 coils is
L1(a), L2(a), L3(a) . . . repeated 5 times around the circle. With
this connection, the machine rotates at 1/5.sup.th the frequency of
the (a) excitation, that is at 720 RPM for 60 Hz. The (b)
excitation repeats three times around the stator, so must be at a
frequency of 36 Hz to correspond to the same rotation speed of 720
RPM.
[0063] The (a) and the (b) excitations do not couple to each other
as long as L1(b)+L2(b)+L3(b)+L4(b)+L5(b)=0. This is true if the (b)
excitation is a 5-phase sinusoidal waveform but not if it is a
5-phase squarewave. However, a 5-phase modified square wave may be
used if, at every point in time, two of the signals are +V volts,
two of the signals are -V volts and a fifth is zero, as illustrated
by the waveforms of FIG. 12. Such a waveform is known as a modified
squarewave and is also known as a modified sinewave in the special
case that it has the same peak and RMS values as a sinewave. For
orthogonality, it is actually the L1(b) . . . L5(b) currents that
should sum to zero, and this can be approximately arranged using a
5-phase version of the transistor switching circuit of FIG. 4 by
having both transistors of a pair such as (Q1, Q4) turned off when
that phase is desired to be zero rather than positive or negative.
The MOSFET transistors have intrinsic drain-source diodes which
will catch any back-EMF from opening a stator coil, thus preventing
damage; however, this will also result in the sum of the 5-phase
current not being perfectly zero at all times, but exhibiting
glitches which will feed into the (a) excitation.
[0064] Even when it is not necessary to create such 3-level
waveforms, it can be advantageous to have a short period when both
transistors of a pair are off when switching polarity. This so
called "notching" of the drive waveforms reduces current
transients.
[0065] The condition for the (a) excitation phase currents to sum
to zero can be more easily and accurately met when the number of
(a) phases is even, allowing half of them to be positive, and the
other half to be negative, eliminating the need for a zero current
level.
[0066] FIG. 10 shows an arrangement of 12 stator coils fed with a
12-phase (b) excitation at their outer ends and connected to a
3-phase excitation at their inner ends. As previously discussed,
the number of stator coils is preferably even to facilitate
maintaining orthogonality with a squarewave (b) excitation, and
also must be a multiple of the number of (a) phases, i.e. three.
Thus the number of poles/stator coils should be 2.times.3.times.N
where N is any integer. In this case it is substantially just as
easy to produce a 12-phase excitation from switching transistors as
a lower number of phases, so a number of (b) phases equal to the
number of stator coils may as well always be used.
[0067] A 12-phase excitation has not only the property that
L1+L2+L3+L4+L5+L6+L7+L8+L9+L10+L11+L12=0 (1)
but also the properties that
L1+L7=L2+L8=L3+L9=L4+L10=L5+L11=L6+L12=0 (2)
L1+L3+L5+L7+L9+L11=0 and L2+L4+L6+L8+L10+L12=0 (3)
L1+L4+L7+L10=L2+L5+L8+L11=L3+L6+L9+L12=0 (5)
and
L1+L5+L9=L2+L6+L10=L3+L7+L11=L4+L8+L12=0 (6)
but this latter property is true only for modified squarewaves,
while properties (1) to (5) hold for unmodified squarewaves.
[0068] FIG. 10 uses the property of equation (5) above to maintain
orthogonality between the (a) and the (b) excitations when the (b)
excitation consists of square waveforms. It may be seen that the
coils receiving L1(b), L4(b), L7(b) and L10(b) excitation at their
outer ends are connected at their inner ends (by the solid
connecting lines) to the L1(a) excitation. This is in effect a
neutral point for the 4-phase excitation L1(b), L4(b), L7(b),
L10(b) and thus no (b) excitation is coupled to L1(a). Likewise the
dashed connecting lines couple L2(b), L5(b), L8(b)L11(b) to L2(a)
and the dotted connecting lines couple L3(b), L6(b), L9(b)L12(b) to
L3(a).
[0069] As the (a) excitation repeats four times around the stator
coils, the rotation speed is 1/4 that of the (a) excitation
frequency, namely 900 RPM for 60 Hz The (b) excitation only has one
cycle around the stator coils so the (b) excitation frequency must
therefore be 900 RPM/60 seconds=15 Hz.
[0070] The (a) excitation may be a WYE or DELTA connection to a
3-phase utility supply, and no (b) excitation current now flows in
the utility service drop transformers, so they do not need to be
over-dimensioned.
[0071] When adjacent stator coil windings overlap as in FIG. 8, the
mutual coupling between coils due to overlap is constructive to the
magnetic field when adjacent phases of an excitation are less than
90 degrees apart. This pertains when using the alternative
arrangement of FIG. 11. In FIG. 11, the 3-phase utility connection
uses split-phase service drop transformers for each phase to obtain
phase and phase+180 degrees, making 6 phases in total. This
provides a 6-phase (a) excitation with 60 degrees difference
between adjacent coils. Orthogonality with the (b) excitation
relies upon equation (2) above; opposite pairs of coils, such as
those excited out of phase at their outer ends by L1(b), L7(b), are
connected together at their inner ends to the zero-phase (a)
excitation L1(a). The -L1(a) excitation from the other phase of the
L1(a) utility transformer has a phase of 180 degrees and is
connected to the inner ends of the coils which are excited at their
outer ends by L4(b), L10(b). The other coils are similarly
connected to the L2(a) and L3(a) utility transformers such that the
(a) excitation produces the phase progression 0, 60, 120, 180, 240,
300 repeating twice around the stator. The rotation speed is thus
1800 RPM when the (a) excitation frequency is 60 Hz, so the
12-phase (a) excitation, which repeats only once around the stator,
must then be 30 Hz plus the necessary slip rate. As with FIGS. 9
and 10, no (b) excitation current flows in the utility transformer
windings, so they need not be over-dimensioned; however current
delivered to the utility transformers when the machine is
generating also flows in the switching transistors that produce the
(b) excitation, so the transistors must be sized accordingly to
handle both the (b) current and some of the (a) current.
[0072] When the (b) current is derived from a solar array, no
overcurrent protection on the (b) excitation side of the machine is
necessary, as solar arrays are current limited; overcurrent
protection on the (a) side may appropriately be provided by using a
2-pole breaker on the 240 volt connections to each of the utility
transformers (1010). It is immaterial if one of these two-pole
breakers trips and not the other two. The machine will continue to
deliver current to the remaining two phases as long as it does not
exceed their breaker ratings. If two 2-pole breakers trip, the
machine will still function as a single phase generator and 12 pole
motor.
[0073] For completeness, FIG. 13 shows how the same 12-pole machine
of FIGS. 10 and 11 can be connected to a single split-phase utility
transformer, relying upon equation (3) to maintain orthogonality
between the (a) and (b) excitations.
[0074] In FIG. 13, the odd numbered stator coils have their inner
ends connected to the 0-degree leg of the utility service drop
transformer (1010) while the 180-degree leg connects to the inner
ends of the even numbered coils. A two phase (b) excitation (0,
180) is thus produced which repeats 6 times around the stator. The
rotation frequency is thus 60 Hz.times.60 Seconds/6=600 RPM. In
this case the (b) excitation is a 12 phase signal at 10 Hz.
[0075] It will be appreciated that, when there is a ground
connection on the machine side of the utility transformers, DC
input 2020 of FIG. 4 must be floating to allow it to supply equal
positive and negative currents to the stator coils. A floating
solar array is contemplated as the DC source, as was also a feature
of the invention described in allowed U.S. patent application Ser.
No. 13/103,070 to current inventor, which is incorporated by
reference herein. In the '070 Application, it is described how
ripple that can be a harmonic of the switching frequency can appear
as a common mode signal on the array DC lines, and this can also
arise in the current invention. Such a ripple, if a consistent
waveform, can be useful in detecting ground leaks on the DC side.
In the above-incorporated Application, the design of common-mode
filters to prevent the export of switching transients is also
described, and this can be part of filter 2010 of FIG. 4.
[0076] FIG. 4 can be expanded by adding further switching
transistor pairs such as (Q1, Q4) to provide as many phases as
desired, such as the 12 phases for FIGS. 10, 11 and 13. Multiple
phases of a squarewave may be produced by starting with a clock
frequency that is a multiple of the desired excitation frequency
F(b) times the number of phases N. For example, a clock frequency
of 2NF(b) is suitable. This frequency is first divided by N to
produce 2F(b) and then divided by 2 to produce a squarewave at F(b)
with accurate 50/50 mark-space ratio. The squarewave is then
delayed in a shift register clocked at NF(b) to produce squarewaves
delayed by multiples of 1/Nth of a cycle. It may also be mentioned
that two-level waveforms other than squarewaves can be used; for
example Pulse Width Modulated waveforms, which preferably comply
with whichever of equations (1) to (6) above is being relied upon
for orthogonality between excitations (b) and (a)
[0077] F(b) is the frequency of the (b) excitation and must be
slightly faster, by a determined amount dF, than an integral
multiple or sub-multiple of the grid frequency in order to transfer
power from the DC source to the grid, the amount of power
transferred being proportional to dF. Either analog circuits, such
as a voltage controlled oscillator, or digital circuits, such as a
Direct Digital Frequency Synthesizer, may be used to produce the
frequency 2NF(b) from which F(b) can be derived as described above,
and to control it to achieve the desired slip frequency dF.
[0078] Increasing the slip frequency increases the current drawn
from the DC source. When the DC source is a solar array, an
increase in current drawn causes the voltage to fall, but there is
an optimum current Imp and corresponding voltage Vmp at which the
array delivers its maximum power Vmp.times.Imp. Control circuit
(2000) of FIG. 4 thus may be designed to control the frequency
while monitoring the array voltage and/or current until it is
operating at the maximum power point approximately. The maximum
power point voltage Vmp is not very dependent on illumination level
but is more dependent on temperature. A temperature sensor can be
used along with a prestored curve of Vmp versus temperature to
determine the target operating point. Other methods can be used,
for example providing one extra solar cell that is monitored by a
pilot circuit that sweeps the V/I curve of the extra cell to
determine Vmp for that cell. This is then scaled by the number of
cells in an array string to determine what Vmp should be for the
array. Such maximum power point tracking operates continuously to
adapt to changes in illumination caused by clouds passing over. Of
course there are many elements of a complete practical installation
that have not been described in detail as they are immaterial to
the invention; for example, appropriate overcurrent protection
devices should be used and metering of the power delivered to the
grid would be included for financial accounting purposes. It is
also known that, due to the high inertia of big machines, starting
or spin-up arrangements may be needed to avoid excessive starting
currents. In the current invention, spin up may be advantageously
done by powering the machine from the solar array alone, with the
utility disconnected. When the machine is up to speed, the utility
power may first be connected through a resistive elements such as
lamps and if everything seems to be working normally, the lamps are
then shorted out. Alternatively the machine may be spun-up using
utility power with the aid of any of the prior art large motor
starting arrangements.
[0079] Many other variations and adaptations of the invention
described above can be devised. For example, two or more smaller
machines can be operated together using the same or separate DC
sources and their outputs both fed to the grid. Moreover, their
shafts can be coupled or not, to achieve different attributes. For
example, two machines of the FIG. 3 type with or without coupled
shafts can be operated from separate switching circuits (1000) that
are 180 degrees out of phase, thereby cancelling the (b) excitation
current fed to the utility transformers (1010), which do not then
need to be over-dimensioned. An arrangement of two or more machines
with coupled shafts and appropriately-phased(b)-excitations may
also be used to cancel some current waveform distortions that can
arise due to magnetic material saturation.
[0080] It was also mentioned that the low switching rate needed for
the (b) excitation can in some cases lend itself to the use of
mechanical switches rather than transistors. Such variations,
including variations in the number of poles, stator winding slots,
rotor winding slots and excitation phases that may be devised using
the teachings herein fall within the scope of the claimed invention
as described by the attached claims.
* * * * *